A seamless steel pipe, which is more reliable than a welded pipe, is frequently used in a sever oil well environment or high-temperature environment, and the enhancement of strength, improvement in toughness and improvement in sour resistance are therefore consistently required. Particularly, in oil wells to be developed in future, the enhancement in strength of the steel pipe is needed more than ever before because a high-depth well will become the mainstream, and a seamless steel pipe for oil wells also having stress corrosion cracking resistance is increasingly required because the pipe is used in a severe corrosive environment.
The hardness, namely the dislocation density, of steel product is raised as the strength is enhanced, and the amount of hydrogen to be penetrated into the steel product increases to make the steel product fragile to stress because of the high dislocation density. Accordingly, the sulfide stress cracking resistance is generally deteriorated against the enhancement in strength of the steel product used in a hydrogen sulfide-rich environment. Particularly, when a member having a desired yield strength is produced by use of a steel product with a low ratio of “yield strength/tensile strength” (hereinafter referred to as yield ratio), the tensile strength and hardness are apt to increase, and the sulfide stress cracking resistance is remarkably deteriorated. Therefore, when the strength of the steel product is raised, it is important to increase the yield ratio for keeping the hardness low.
Although it is preferable to make the steel product into a uniform tempered martensitic microstructure for increasing the yield ratio of the steel, that alone is insufficient. As a method for further enhancing the yield ratio in the tempered martensitic microstructure, refinement of prior-austenite grains is given. However, the refinement of austenite grains needs quenching in an off-line heat treatment, which deteriorates the production efficiency and increases the energy used. Therefore, this method is disadvantageous in these days where rationalization of cost, improvement in production efficiency and energy saving are indispensable to manufacturers.
It is described in the Patent Documents 1 and 2 that precipitation of a M23C6 type carbide in grain boundary is inhibited to improve the sulfide stress cracking resistance. An improvement in sulfide stress cracking resistance by refinement of grains is also disclosed in the Patent Document 3. However, such measures have the difficulties as described above.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-73086,    Patent Document 2: Japanese Laid-Open Patent Publication No. 2000-17389,    Patent Document 3: Japanese Laid-Open Patent Publication No. 9-111343.